It is among the objectives of modern materials science to develop coatings which have improved strength, flexibility, tolerance to hostile environmental conditions, and improved gas- and liquid-barrier properties.
Recent advances in polymer technology have provided new materials which have improved properties.
Rod-like, extended-chain, aromatic-heterocyclic polymers have received considerable interest in both academic and industrial laboratories over the past two decades. These "ordered" polymers include "thermotropic" liquid crystalline polymers, which are modified by temperature changes. Thermotropic LCPs are of great interest because they exhibit a partially ordered state that is intermediate between a three-dimensional ordered crystalline state, and the disordered or isotropic fluid state. As a consequence of their molecular ordering, LCPs are anisotropic, i.e. their properties are a function of molecular direction (R. A. Weiss and C. K. Ober, "Liquid Crystalline Polymers," A.C.S. Symposium Series 435 (1990)).
Structurally many commercial LCPs consist of rigid mesogenic monomer units connected with either flexible spacers or "kink structures" to make them tractable and processable. The high degree of molecular order that can be achieved with the LCP molecules allows this material to attain a very tight packing density, similar to a log jam in a river. LCPs derive their outstanding properties from this tightly packed rigid-rod formation which at a macroscopic level results in a structure that is self-reinforced through the strong interaction of electron-deficient and electron-rich benzene rings.
Because of their rigid backbone structure with flexible spacer groups, commercially available thermotropic LCPs have far higher tensile strength and flexural moduli than conventional polymers.
One important application for coating technology is the coating of optical fiber used in the telecommunications and aerospace industries. Ultraviolet-cured epoxy acrylate (UVEA) has emerged as a frequently used material because it can be successfully applied during the high-speed optical fiber drawing (manufacturing) process. While UVEA is satisfactory for many telecommunications applications, there are other applications for which UVEA is not optimal. For example, UVEA does not satisfy the requirements of rapidly deployable data links, which are used for transmitting guidance signals in aerospace applications. In order to prevent water-induced stress corrosion on an optical fiber, one typically needs to use UVEA coating 3 mil (0.0762 mm) thick which is heavier than a 1 mil (0.0254 mm) TLCP coating and also bulkier, resulting in a limited length of coated fiber which can be accommodated on a spool, carried on an aircraft for payout. This means the object to be guided can be guided only for a shorter distance. Further, UVEA has higher surface friction than TLCP, which means that UVEA tends to cause more "sticking" or "blocking" during unspooling than TLCP.
Although thermotropic LCPs possess a variety of properties that make them an attractive candidate for coating purposes, standard LCP processing results in material with uniaxial orientation (all molecules aligned in one direction). Such materials have very good machine (extrusion) direction (MD) mechanical properties and very poor transverse direction (TD) mechanical properties. If standard extrusion techniques are used to extrude LCPs over a tubular structure, the resulting LCP coating with its axial molecular orientation would readily split in the axial direction, when exposed to bending. For example, conventional uniaxial TLCP coatings, when applied to a flexible optical fiber member, are likely to split along the fiber axis when subjected to bending. Such uniaxially oriented material also has a highly negative coefficient of thermal expansion ("CTE") in the extrusion direction and a highly positive CTE in the transverse direction. This characteristic is generally unacceptable for coating members which will be exposed to thermal cycles. In such cases, it is desirable to have the CTE of the coating more closely approach the CTE of the member.